The present invention relates to electromagnetic radiation shielding composites which exhibit high conductivity and low resistance.
As technology progresses and produces more sophisticated electronic equipment, the environment becomes more polluted with electromagnetic (EM) radiation. This radiation has recently become recognized as constituting a significant hazard to the health of individuals as well as to the operation of electronic equipment, and in particular, to digital equipment that is generally more easily adversely affected by spurious radiation. There is therefore a need to control such random and undirected radiated electromagnetic energy while also shielding the environment (including both biological life and equipment) from its effects.
The shielding of both emitted radiation and incoming radiation has traditionally been done by metallic enclosures. Metal serves as a shield as a result of its high conductivity due to the fact that the penetration of EM radiation is dependent upon the conductivity exhibited by the shield. In fact, continuous metal enclosures having a thickness in the range of 1/32 inch to 1/10 inch serve as effective shields for EM radiation over the radio-frequency band from the kilohertz to gigahertz range. The most common metallic enclosures are comprised of steel, aluminum and copper. However, such metallic shielding enclosures are unfortunately both heavy and costly.
As an alternative, equipment has been enclosed in plastic structures consisting of polymeric moulding compounds which have dispersed therein conductive materials such as metallic fibers, carbon fibers, etc. See for example, U.S. Pat. No. 3,996,167, issued to Brown. Composites have also been modified to produce higher conductivity by the use of surface treatments such as conductive paints, spray plating, flame spraying, or vacuum metallizing. See, for example, U.S. Pat. No. 2,321,587, issued to Davie et al. However, these techniques often either prove to be too costly or do not sufficiently increase the conductivity of the structure to provide the desired amount of shielding.
Due to the flexibility of design provided by utilizing molded plastic parts as shielding composites, it is desirable to provide adequate electromagnetic shielding by incorporating metallic or other conductive materials (e.g., in flake or fiber form) into the polymeric matrix to eliminate the use of secondary coating operations. A further advantage of integrally incorporated particles is that control of the passage of electromagnetic waves is achieved by absorption as well as by reflection because of the bulk conductivity created.
However, there is presently a need for an efficient and economical means of providing shielding which exhibits minimal resistance and high conductivity. Due to the fact that only limited amounts of conductive materials can be incorporated into such shielding composites without imparing the ability of the matrix material from which the composite is made to be molded, it is desired to provide a method to increase the conductivity of the composite without having to unnecessarily increase the amount of conductive material employed.
In addition, the use of shielding composites comprised of structural foams is advantageous in certain instances where a lower weight composite is required. The maximum amount of conductive material which the foamed composite can contain may be less than the maximum amount which an unfoamed composite can contain since less effective volume may be available within which to disperse the material.
Therefore, it is desirable to provide a method by which the conductivity of such shielding composites can be increased over and above that exhibited by the composite subsequent to incorporation of the conductive material.